1. Field of the Invention
This invention relates to a direct-coupling control system for torque converter in an automatic transmission for vehicles.
2. Description of Prior Art
In vehicular automatic transmission equipped with a hydraulic torque converter, it is preferred, in view of fuel economy, power efficiency and serenity, to mechanically directly couple or lock up input and output members of the torque converter together in such a state that its torque amplifying function is not desired and/or is not substantially obtainable. It may be further preferable that such lockup is effected not only in the highest stage but also in a low speed stage. The problem, however, exists in that the shock caused at the time of gear shifting may be enlarged undesirably unless the lockup state is fully released.
In case of systems in which the speed change and lockup are electronically controlled, it is possible to effect the lockup again by releasing the formerly established lockup state prior to the speed change operation after the lapse of a certain time with a speed change command having been given. This enables a sufficient time to be ensured for releasing operation so that the aforementioned problem of increasing the shock caused by gear shifting would not occur; however, the production cost will increase and the system will be complicated.
On the other hand, if the speed change and lockup are hydraulically controlled, the system will be simple and less costly. However, there has not yet been established a method for detecting the speed change operation beforehand. For this reason, the lockup is released simultaneously when the speed change operation is effected so that the release of lockup as well as the relockup must be performed within an extremely short time of speed change operation. In order to release the lockup within such a short time, the engaging force of the direct-coupling mechanism is so arranged as to be determined by the function of pressure differential between the internal pressure and working pressure of the torque converter to make the above-described internal pressure always act on a released side. With this arrangement, the lockup can be released with excellent response characteristics only by reducing the working pressure when the speed change operation is effected. Although the present assignee has already filed an application for a patent and disclosed therein such a release mechanism, there has still been an unsolved problem that in such release mechanism, the pressure range which can be utilized as the pushing pressure of the piston will be reduced if the internal pressure of the torque converter is preset high, resulting in that a sufficient engaging force cannot be secured unless a sufficient area is provided on the piston. Accordingly, there may be considered cases where the internal pressure of the torque converter is set at a value slightly smaller than the vehicle speed or is set to decrease in proportion thereto. However, this would result in an increased shock since the lockup is insufficiently released at the time of speed change operation as the internal pressure of the torque converter is decreased.
If the throttle pedal is released to reduce the speed to adjust the interval from a preceding vehicle during cruising or accelerating, it will be preferred to release the lockup as quickly as possible for avoiding the awkwardness and saving the quantity of fuel to be sucked by the carburetor when the speed is being reduced. Accordingly, even if the internal pressure of the torque converter is set rather low, it will be desirable that the lockup state may be released with a quick response by increasing the internal pressure of the torque converter when the throttle pedal is returned to the idle position.
If the actuating pressure applied to the hydraulic clutch is reduced by the increased amount of internal pressure in association with increasing of the internal pressure in the torque converter as mentioned above when the throttle pedal is returned to the idle position, the speed changing operation will be slowed down and the speed changing shock will be mitigated.